(1) Field of the Invention
The present invention relates to a stabilizing agent for a peroxide-bleaching procedure and methods of bleaching a fiber material by using the stabilizing agent. More particularly, the present invention relates to a silicate-free stabilizing agent for a peroxide-bleaching procedure and methods of bleaching a fiber material with a peroxide-containing bleaching agent under a stabilized condition, by preventing decomposition of the bleaching agent by impurities, for example, heavy metal ions and alkaline earth metals, introduced from the fiber material and industrial water into the bleaching system, with the silicate-free stabilizing agent.
The term "a fiber material" used herein includes fiber masses, fine fibrous particles, slivers, tows, yarns, webs, tapes, sheets (woven, knitted and nonwoven fabrics), and shaped articles comprising at least one type of natural organic and inorganic fibers, and wood and non-wood pulps.
(2) Description of the Related Art
It is well-known that conventional chlorine-containing bleaching agents, for example, chlorine gas and hypochlorous acid salts, are cheap and have a strong bleaching activity for various fiber materials and paper-forming pulps. However, the chlorine-containing bleaching agents are disadvantageous in that they per se are dangerous in corrosion of the skin and apparatus and in production of harmful substances, for example, dioxins and chloroform. Currently, the chlorine-containing bleaching agents are, therefore, being superseded by oxygen-containing bleaching agents, for example, oxygen gas and peroxo compounds.
The conventional bleaching method using the oxygen-containing bleaching agents will be explained below by taking a bleaching method using hydrogen peroxide as an example.
Generally, a bleaching method using hydrogen peroxide is carried out under an alkaline condition. The alkali is preferably selected from sodium hydroxide and sodium carbonate. When the hydrogen peroxide-bleaching procedure is carried out under alkaline conditions and the bleaching system contains some heavy metal ions, for example, Mn and Fe ions, hydrogen peroxide is rapidly decomposed in the presence of the heavy metal ions. Therefore, to enhance the bleaching efficiency, the decomposition of hydrogen peroxide has to be prevented by adding a stabilizing agent to the bleaching system. Usually, sodium silicate is used as a decomposition-preventing agent for hydrogen peroxide.
Sodium silicate is advantageous in its low price and high stabilizing effect on hydrogen peroxide. However, when sodium silicate is added to a bleaching system containing multivalent metal ions, for example, calcium and magnesium, it causes a deposition of water-insoluble silicate scale on the surfaces of individual fibers in the fiber material and the inside surfaces of the bleaching apparatus, the scaled fiber material exhibits a bad hand feeling and a degraded sewing property, and the scales on the inside surfaces of the bleaching apparatus damage the individual fibers in the fiber material. Sometimes, the individual fibers are broken by the scales. These phenomena is referred to as silicate obstruction.
Also, in production of paper and pulp using sodium silicate, the silicate scale causes stoppage of pipelines and machine, clogging of wire nets and staining of dryer. These phenomena also cause hole-formation on the resultant paper sheets, insufficient water removal by the paper-forming blanket and staining of the paper-drying canvas.
Recently, for the purpose of reducing a consumption of fresh industrial water in response to supply shortages of industrial water and of preventing environmental pollution due to waste water discharged from the paper and pulp-producing factory, it has been attempted to introduce a closed water-recycling system in which the discharge of the waste water is restricted to the utmost. The closed system is now practically utilized in some factories. When sodium silicate is used in a closed bleaching system, the resultant water-insoluble silicate is accumulated in the bleaching system, and deposited on the inside surfaces of the bleaching vessel and pipelines and thus causes water recycling through the system to be affected.
To prevent the silicate obstruction, it has been attempted to replace the sodium silicate by a non-silicate type organic metal-chelating agent.
Japanese Examined Patent Publication No. 60-1,360 discloses that poly-.alpha.-hydroxyacrylic acid salt (PHAS) is an excellent stabilizing agent for hydrogen peroxide used as a bleaching agent. PHAS is, however, disadvantageous in that when a concentration of heavy metal ions, for example, manganese (Mn), iron (Fe) and copper (Cu) ions, especially manganese ions, introduced in the bleaching system fluctuates, the PHAS cannot follow the fluctuation and thus sufficiently stabilize the bleaching system. Therefore, the bleaching effect by hydrogen peroxide cannot be kept sufficiently constant. In the bleaching system for pulp, the concentration of the heavy metal ions, for example, Mn, Fe and Cu ions, always fluctuates due to change in type of tree for the pulp and in the composition of the industrial water. Accordingly, the PHAS is unsatisfactory as a stabilizing agent for practical pulp-bleaching systems.
To solve the above-mentioned problem due to the heavy metal ions, Japanese Unexamined Patent Publication No. 5-148,784 provides a bleaching process in which lignocellulose-containing pulp material is pre-treated with an aqueous acid solution at a pH value of 1 to 6 and then with an aqueous solution of an alkaline earth metal-containing compound at a pH value of 1 to 7, and bleached with ozone or a peroxo compound, and Japanese Unexamined Patent Publication No. 5-148,785 provides a bleaching method in which a lignocellulose-containing pulp material is pre-treated with an aqueous solution of nitrogen-containing carboxylic acid-complexing agent at a pH value of 3.1 to 9.0 and then bleached with ozone or a peroxo compound.
The pre-treatments disclosed in the Japanese publications are unsatisfactory in heavy metal-removal effect. Further, in the bleaching procedure with the peroxo compound of the above-mentioned processes, sometimes, an additive selected from, for example, magnesium-containing compounds must be added to the bleaching solution, to control the physical properties, for example, viscosity, of the bleaching solution.
Further, alkaline earth metals, for example, magnesium (Mg) and calcium (Ca) are introduced from the pulp material and industrial water into the bleaching solution, and thus the concentration of the alkaline earth metals in the bleaching solution fluctuates due to the industrial water and the pulp material. The alkaline earth metals per se do not promote the decomposition of hydrogen peroxide. However, these metals react with a chelating agent added as a stabilizing agent for hydrogen peroxide to the bleaching solution and cause the stabilizing function of the chelating agent to be reduced or lost. Accordingly, the stability of the bleaching solution containing hydrogen peroxide is significantly reduced with an increase in the concentration of the alkaline earth metals in the bleaching solution.
Usually, a bleaching procedure with a peroxide bleaching agent, for example, hydrogen peroxide is carried out in a range of pH values from 8 to 12. Therefore, a stabilizing agent for a peroxide bleaching procedure is required to be constantly effective over the range of pH values 8 to 12. However, the stabilizing effect of the conventional stabilizing agent varies depending on the pH value of the peroxide bleaching system.
Accordingly, there is a strong demand for a new type of stabilizing agent capable of exhibiting a constant high stabilizing effect for a peroxide bleaching procedure over a range of the pH values from 8 to 12.